GLOBAL warming could cause the bottom layers of many deep freshwater lakes to stagnate, including the Great Lakes of North America, Scandinavian lakes and some of Scotland鈥檚 lochs. If this happens, many deep-water fauna that have thrived in these lakes for 15,000 years could be in danger.
The lakes鈥� thermal structure is such that the surface and deep water differ in temperature and density. To keep the bottom layers fresh, these layers have to mix, and for this to occur, they have to reach a similar temperature and density. This normally happens in winter, when the surface waters cool.
But Ulrich von Grafenstein of the Laboratory of Climate and Environmental Sciences (LSCE) in Gif-sur-Yvette, France, and his colleagues found that if the climate warms as predicted, the surface waters won鈥檛 cool sufficiently for the layers to mix.
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The researchers modelled how climate change would affect the circulation in two deep freshwater lakes: Lake Ammersee in Germany and Lake Annecy in France. They found that just a 1 掳C rise in air temperature was enough to upset Ammersee鈥檚 thermal structure and prevent it from overturning and mixing. 鈥淏y 2020 we expect Lake Ammersee to develop a stagnant bottom layer,鈥� says von Grafenstein.
Lake Annecy may escape this fate. Because it is a little shallower and has clearer waters than Ammersee, sunlight can penetrate further and warm the deep water, bringing its density closer to that of the surface water (Geophysical Research Letters, DOI: 10.1029/2004GL020833).
However, the outlook for deep-water species is bleak in both lakes. 鈥淚f the deep waters warm too much in Lake Annecy then the Arctic char will not be able to survive because its eggs cannot develop at temperatures above 7 掳C,鈥� explains von Grafenstein. Meanwhile, all the bottom water species in Lake Ammersee, including deep-dwelling crustaceans and the Ammerseerenke fish, will die from lack of oxygen.
Hundreds of other freshwater lakes around the world have a similar structure and local climate. Von Grafenstein fears that most of these lakes will eventually be damaged by global warming. 鈥淓uropean lakes like Ammersee will see the effect of global warming first, but lakes in colder climates are likely to experience the same problems later too,鈥� he says.
Not everyone is convinced. 鈥淟ake overturning also depends on the wind and I don鈥檛 think our ability to forecast the wind in the future is very good,鈥� says Michael McCormick, a lake circulation expert at the Great Lakes Environmental Research Laboratory in Ann Arbor, Michigan. 鈥淚t is possible that global warming could stop some lakes from overturning, but I鈥檓 not sure how likely it is.鈥�
